5.3.1 Suppressing Network Connections

In some circumstances it is desirable to suppress making network
connections. A typical case is when rendering HTML in a mail user
agent, when external URLs should not be activated, particularly to
avoid “bugs” which “call home” by fetch single-pixel images and the
like. To arrange this, bind the following variable for the duration
of such processing.

Variable: url-gateway-unplugged

If this variable is non-nil new network connections are never
opened by the URL library.